1. Field of the Invention
The present invention relates to a color display device and a color display method for reproducing color with an increased number of gradation levels, and more specifically to the configuration of sub-pixels forming a pixel and the generation of image data to be supplied to the sub-pixels in a color display device such as a liquid crystal display device. In particular, the present invention relates to the configuration of sub-pixels for reproducing subsidiary colors for compensating, for example, the three RGB (red, green and blue) primary colors and the generation and transfer of the subsidiary color data.
2. Description of the Background Art
Generally, a color display device such as a liquid crystal display (LCD) device is made up of a number of pixels or picture elements each of which is composed of three sub-pixels corresponding to the three primary colors, RGB, in order to represent the color of a pixel by a combination of the three primary colors. In this connection, International Publication No. WO 03/088203 A1 to Roth et al., discloses a technique of expanding the range of color reproducible by the use of one or more primary color(s) in addition to the RGB colors. Examples of such an additional primary color or colors may be yellow, cyan and/or magenta.
In this case, the number of possible colors is determined by the number of different gradation, or intensity, levels which can be reproduced by each sub-pixel. For example, in the case where each sub-pixel can reproduce 256 different gradation levels, thus by eight bits, the number of possible colors reproduced by the RGB (i.e. three) sub-pixels is equal to 256 cubed, i.e. about 16.7 millions in combination. In general, the gradation level of each of the sub-pixels is controlled by varying the voltage applied to the corresponding LCD cell to adjust the optical transmittance of that sub-pixel. This voltage is applied by a driving device. In the following, such a driving device is referred to as an LCD driver. Needless to say, the number of colors which can be reproduced increases as the number of gradation levels increases.
The number of gradation levels is determined by how fine the steps, i.e. the resolution, of a driver voltage an LCD driver supplies are designed.
If the resolution is increased, then the number of gradation levels, or colors reproducible, can be increased. However, for example, if the number of gradation levels is increased from 256 different gradation levels, requiring eight bits, to 1024 different gradation levels, requiring ten bits, then the number of circuit components such as selectors of the LCD driver increases in proportion to the number of gradation levels so that the circuitry of the LCD driver becomes complicated to increase the burden on designing, manufacturing and so forth. Furthermore, in this case, the voltage step per gradation, i.e. the step defined by the least significant bit (LSB), is decreased so that the deviation in voltage is more restrictive.
Another problem is related to the requirement that the three RGB primary colors are to be differently controlled in the case of the LCD device. This is because the three RGB primary colors have their own optical transmittance versus voltage characteristics respectively. Accordingly, it is necessary to provide the information about three curves indicating the transmittance versus voltage characteristics in correspondence with the respective, three RGB primary colors.
However, if the three RGB primary colors are finely or individually controlled with respect to the gradation levels, the circuit scale of the LCD driver is significantly increased, and thereby the requirement for high accuracy in controlling the gradation voltages entails an increase in production costs.
Taking into consideration the above circumstances, it is an object of the present invention to provide a color display device and a color display method in which the number of gradation levels can be substantially increased without increasing the number of gradation levels of the primary colors, e.g. without enhancing the resolution of the voltage applied to the color display panel by the LCD driver.
Furthermore, there is the following problem in the technique disclosed by Roth et al., stated above. While a color display device reproduces an arbitrary color by a combination of the three RGB primary colors in usual cases, it is proposed in the above publication to use a subsidiary color or colors in addition to the three RGB primary colors in order to reproduce with a high degree of accuracy particular colors which cannot accurately be reproduced only by a combination of the three RGB primary colors. In this case, the subsidiary color is attained by providing a color filter on a subsidiary pixel of the liquid crystal panel. RGB data is generated by converting a luminance signal and color-difference or chrominance signals by a graphics processor which receives or generates the image data to be displayed on the liquid crystal display device. The graphics processor outputs the RGB data to a timing controller. The timing controller arrays pixel values of the RGB data onto each horizontal line and transfers them to the LCD driver provided on the LCD panel. The timing controller also generates other signals necessary for displaying an image on the liquid crystal display device.
The luminance signal and color-difference signals can be converted into the RGB data, for example, in accordance with the following expressions.R=1.164*(Y−16)+1.596*(Cr−128)G=1.164*(Y−16)−0.391*(Cb−128)−0.813*(Cr−128)B=1.164*(Y−16)+2.018*(Cr−128)
In the above expressions, Y is a luminance signal, and Cr and Cb are color-difference signals. By this conversion, the color information out of the RGB gamut is substantially discarded from the luminance signal and the color-difference signals. This is referred to as clipping. A subsidiary color can be generated from the data which is discarded by the clipping. However, the discarded data as it is cannot be used as data of the subsidiary color. A very complicated algorithm is necessary for making use of the discarded data as the subsidiary color.
Taking into consideration the above circumstances, it is also an object of the present invention to provide a color display device and a color display method in which data required for driving the subsidiary sub-pixels can be generated by a simplified process.